Xbox Memberships run anywhere from $15 - $50 or more
If you buy 1-years worth for $50 you get 1-month FREE (a $8 value)
ALWAYS Buy these cards from a retail store, never from eBay, Amazon, etc.
When buying at the store make sure the back of the card HAS NOT been scratched off yet, I have personally purchased cards that were already used by mistake and the store wouldnt refund me nor would Xbox.
Now,
Here are a few links, about benefits of Gold vs. Silver (obviously you'll be buying Gold, unless you want a trial of XBL first)
Membership info.
http://www.xbox.com/en-US/live/memberships/?WT.svl=nav
About the cards.
http://www.xbox.com/en-US/live/memberships/subscriptioncards.htm
Getting started, how to set up accounts, etc.
http://www.xbox.com/en-US/live/gettingstarted101.htm
Much luck to you, enjoy the online expeirence, see ya